(a) Field of the Invention
The present invention relates to a display device and a driving method thereof, and in particular, a light emitting display device and a driving method thereof.
(b) Description of Related Art
Recent trends of light-weighted and thin personal computers and televisions sets also require light-weighted and thin display devices, and flat panel displays satisfying such a requirement is being substituted for conventional cathode ray tubes (CRT).
The flat panel displays include a liquid crystal display (LCD), field emission display (FED), organic light emitting display (OLED), plasma display panel (PDP), and so on.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix and displays images by controlling the luminance of the pixels based on given luminance information. An OLED is a self-emissive display device that displays image by electrically exciting light emitting organic material, and it has low power consumption, wide viewing angle, and fast response time, thereby being advantageous for displaying motion images.
A pixel of an OLED includes a light emitting element and a driving thin film transistor (TFT). The light emitting element emits light having an intensity depending on the current driven by the driving TFT, which in turn depends on the threshold voltage of the driving TFT and the voltage between gate and source of the driving TFT.
The TFT includes polysilicon or amorphous silicon. A polysilicon TFT has several advantages, but it also has disadvantages such as the complexity of manufacturing polysilicon, thereby increasing the manufacturing cost. In addition, it is difficult to make an OLED employing polysilicon TFTs for large displays.
On the contrary, an amorphous silicon TFT is easily applicable to a large OLED and manufactured using fewer number of process steps than the polysilicon TFT. However, the threshold voltage of the amorphous silicon TFT shifts over time under a long-time application of a DC control voltage such that the luminance is varied for a given data voltage.
In the meantime, a long time driving of the light emitting element shifts the threshold voltage of the light emitting element. As for an OLED employing an n-type driving TFT, since the light emitting element is connected to the source of the driving TFT, the shift of the threshold voltage of the light emitting element changes the voltage at the source of the driving TFT to vary the current driven by the driving TFT. Accordingly, the image quality of the OLED may be degraded.